For many applications of diode pumped fiber lasers (e.g., material processing such as cutting and welding of metals) continuous wave (“cw”) high-power and high beam quality (near the diffraction limit) are necessary. The fiber geometry is well suited for high-power, multi-kilowatt (“kW”) operation, because the excessive heat can be efficiently removed over the length of the fiber due to the long, thin geometry of the fiber, which allows heat removal from a fiber at a higher rate than from a bulk solid-state laser.
Fiber lasers must be pumped by an external energy source to generate radiation, and, because of the finite efficiency of a fiber laser, the external energy source must supply more power than is emitted from the fiber laser. Thus, efficient and economical coupling of pump energy into the active region of a fiber laser is desirable.
Many solid state lasers have an output wavelength between about 1–2 μm. Lasers with such an output wavelength may be doped with a dopant, such as, for example, Nd, Er, Yb, and Vn. Therefore, the following description assumes a wavelength in this order when describing a fiber laser. However, in the case that the output wavelength differs from this assumption, dimensions of the laser are scaled appropriately with the output wavelength.
In general, a double clad fiber laser typically consists of a single-mode core for guiding the output laser radiation, which is embedded in a multi-mode waveguide region for guiding the pump laser radiation. The multi-mode waveguide region may be embedded in an outer cladding. The multi-mode cladding is on the order of several ten to several hundred micrometers in diameter and carries the light from one or more pump diodes that are distributed, and coupled in, along the side of the fiber (“side-pumped fiber laser”) or are all located at the two ends of the fiber (“end-pumped fiber laser”).
The active regions are smaller in diameter than the cladding regions and carry the lasing dopant. The dopant absorbs radiation at the pump wavelength and creates gain at the output laser wavelength. Pump laser radiation should efficiently penetrate both the multi-mode waveguide region and the active regions to pump the active region.
In order to achieve cw output power on the order of multi-kW, pump power greater than the output power must be absorbed by the active region of the fiber laser. Typically, the pump power is generated by high-power diode lasers. Single-emitter diode laser devices reach cw output powers of 5 Watts. Thus, several hundred, or even thousands, of the single-emitter devices must be efficiently coupled into the fiber laser for multi-kW output powers. However, individual contacting, holding, and fiber coupling thousands of single-emitter devices is difficult and expensive.
Therefore, high-power diode laser arrays, which combine multiple emitters in a single device and yield total output powers of 50 Watts or more, may be chosen to pump high-power solid state lasers. The output aperture of such an array is typically 10 mm×1 μm with a beam divergence along the long axis of about 5–10° and a divergence in the small axis of about 40–70°. While the higher output power of a diode laser array is advantageous over a single emitter device, the highly astigmatic output beam of the array effectively prohibits fiber coupling of such devices without complicated additional optics (so-called microoptics). Even with microoptics, fiber coupling of diode laser arrays is difficult.